Optical coherent communications allow information to be encoded in the phase and/or frequency in addition to amplitude of an optical signal. Such systems allow for complex modulation in which a single symbol can encode two or more bits of information. A transmitter can convert one or more electrical signals into one or more optical signals, and modulate the optical signal onto a carrier for transmission. At high frequencies, the drive voltage available for the modulator is constrained to reduce power dissipation. In addition, the modulator may have a limited extinction ratio, defined as the ratio of the modulator output power between the on and off states. The limited drive voltage and extinction ratio can distort the modulation constellation representing the various symbols encoded in the optical signal.